In order to reproduce color prints such as for printing, photography or copying, the spectrum of light that emerges from the printed colors is determined. The print is illuminated and the reflected light emerging therefrom is detected. A portion of the light directed to the surfaces of the print is absorbed and a certain amount of the light is transmitted through the colorant and reflected back through the colorant by the substrate. In the prior art, measurements are made of the spectral distribution of the illuminant (or more precisely of the reflection from an unprinted substrate), and the spectral reflectance of the printed substrate. The difference is assumed to be due to absorption by the colorant on the substrate.
However, in addition to the effect of the color of the substrate, it is known that the final apparent color of a print also depends on other characteristics of the substrate on which the color is printed or spread. This is true whether the color data for printing is computer generated or generated by scanning a physical image. Thus, when copies are made on both glossy and matte substrates, with the same colorant thickness, their apparent color is different.
Another problem in the printing field is matching the OD of a printed color with some desired value of color. In general, as the color saturation increases reflective methods become less sensitive and less accurate.
FIG. 1 illustrates a standard apparatus and methodology for measuring colors printed on a sheet. A light source 12 illuminates a sheet having a colored layer 14 printed on a sheet 16 at some angle to the normal to the sheet. A detector 18 which views the surface generally from a direction normal to the surface, receives light which passes through layer 14 and which is diffusely reflected 13 from the surface of sheet 16. Light source 12 is set at an angle so as to avoid specular reflection from the surface of color layer from affecting the color measurement. Diffuse reflection 15 from the surface of color layer 14, does affect the measurement. However, this measurement of the diffuse reflection mimics the apparent optical density seen by an observer, since the observer also views this diffusely reflected light. A series of filters is used to separate the color reaching the detector into spectral components, which breakdown is used to determine the apparent OD of each of the process colors required to reproduce the color or to enable preparation of a specially mixed color.
When the OD of a single patch of process or specially mixed color is being measured, a series of filters are sequentially placed between sheet 16 and detector 18. Each of these filters corresponds to one of the process colors and selectively passes the spectral band absorbed by that process color. The identity of the process color being tested can be determined from the filter which gives the lowest output for detector 18. The OD is determined from the amplitude of the light which reaches the detector with the color filter associated with the particular process color. Here again, the effects of specular reflection from the colored layer is avoided, but there is an effect of the diffuse reflection therefrom on the measurement. Of course, if the identity color being measured is known, a priori, as in an in-line densitometer, the measurement may be made immediately with the correct filter.